As used herein, the term “molten metal” means any metal or combination of metals in liquid form, such as aluminum, copper, iron, zinc, and alloys thereof. The term “gas” means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, Freon, and helium, which may be released into molten metal.
A reverbatory furnace is used to melt metal and retain the molten metal while the metal is in a molten state. The molten metal in the furnace is sometimes called the molten metal bath. Reverbatory furnaces usually include a chamber for retaining a molten metal pump and that chamber is sometimes referred to as the pump well.
Known pumps for pumping molten metal (also called “molten-metal pumps”) include a pump base (also called a “base”, “housing” or “casing”) and a pump chamber (or “chamber” or “molten metal pump chamber”), which is an open area formed within the pump base. Such pumps also include one or more inlets in the pump base, an inlet being an opening to allow molten metal to enter the pump chamber.
A discharge is formed in the pump base and is a channel or conduit that communicates with the molten metal pump chamber, and leads from the pump chamber to the molten metal bath. A tangential discharge is a discharge formed at a tangent to the pump chamber. The discharge may also be axial, in which case the pump is called an axial pump. In an axial pump the pump chamber and discharge may be the essentially the same structure (or different areas of the same structure) since the molten metal entering the chamber is expelled directly through (usually directly above or below) the chamber.
A rotor, also called an impeller, is mounted in the pump chamber and is connected to a drive shaft. The drive shaft is typically a motor shaft coupled to a rotor shaft, wherein the motor shaft has two ends, one end being connected to a motor and the other end being coupled to the rotor shaft. The rotor shaft also has two ends, wherein one end is coupled to the motor shaft and the other end is connected to the rotor. Often, the rotor shaft is comprised of graphite, the motor shaft is comprised of steel, and the two are coupled by a coupling, which is usually comprised of steel.
As the motor turns the drive shaft, the drive shaft turns the rotor and the rotor pushes molten metal out of the pump chamber, through the discharge, which may be an axial or tangential discharge, and into the molten metal bath. Most molten metal pumps are gravity fed, wherein gravity forces molten metal through the inlet and into the pump chamber as the rotor pushes molten metal out of the pump chamber.
Molten metal pump casings and rotors usually, but not necessarily, employ a bearing system comprising ceramic rings wherein there are one or more rings on the rotor that align with rings in the pump chamber such as rings at the inlet (which is usually the opening in the housing at the top of the pump chamber and/or bottom of the pump chamber) when the rotor is placed in the pump chamber. The purpose of the bearing system is to reduce damage to the soft, graphite components, particularly the rotor and pump chamber wall, during pump operation. A known bearing system is described in U.S. Pat. No. 5,203,681 to Cooper, the disclosure of which is incorporated herein by reference. U.S. Pat. Nos. 5,951,243 and 6,093,000, each to Cooper, the disclosures of which are incorporated herein by reference, disclose, respectively, bearings that may be used with molten metal pumps and rigid coupling designs and a monolithic rotor. U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, and U.S. Pat. No. 6,123,523 to Cooper (the disclosure of the afore-mentioned patent to Cooper is incorporated herein by reference) also disclose molten metal pump designs. U.S. Pat. No. 6,303,074 to Cooper, which is incorporated herein by reference, discloses a dual-flow rotor, wherein the rotor has at least one surface that pushes molten metal into the pump chamber.
The materials forming the molten metal pump components that contact the molten metal bath should remain relatively stable in the bath. Structural refractory materials, such as graphite or ceramics, that are resistant to disintegration by corrosive attack from the molten metal may be used. As used herein “ceramics” or “ceramic” refers to any oxidized metal (including silicon) or carbon-based material, excluding graphite, capable of being used in the environment of a molten metal bath. “Graphite” means any type of graphite, whether or not chemically treated. Graphite is particularly suitable for being formed into pump components because it is (a) soft and relatively easy to machine, (b) not as brittle as ceramics and less prone to breakage, and (c) less expensive than ceramics.
Three basic types of pumps for pumping molten metal, such as molten aluminum, are utilized: circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby generally equalizing the temperature of the molten metal. Most often, circulation pumps are used in a reverbatory furnace having an external well. The well is usually an extension of a charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of a reverbatory furnace to a different location such as a launder, ladle, or another furnace. Examples of transfer pumps are disclosed in U.S. Pat. No. 6,345,964 B1 to Cooper, the disclosure of which is incorporated herein by reference, and U.S. Pat. No. 5,203,681.
Gas-release pumps, such as gas-injection pumps, circulate molten metal while releasing a gas into the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium, from the molten metal. As is known by those skilled in the art, the removing of dissolved gas is known as “degassing” while the removal of magnesium is known as “demagging.” Gas-release pumps may be used for either of these purposes or for any other application for which it is desirable to introduce gas into molten metal. Gas-release pumps generally include a gas-transfer conduit having a first end that is connected to a gas source and a second submerged in the molten metal bath. Gas is introduced into the first end of the gas-transfer conduit and is released from the second end into the molten metal. The gas may be released downstream of the pump chamber into either the pump discharge or a metal-transfer conduit extending from the discharge, or into a stream of molten metal exiting either the discharge or the metal-transfer conduit. Alternatively, gas may be released into the pump chamber or upstream of the pump chamber at a position where it enters the pump chamber. A system for releasing gas into a pump chamber is disclosed in U.S. Pat. No. 6,123,523 to Cooper. Furthermore, gas may be released into a stream of molten metal passing through a discharge or metal-transfer conduit wherein the position of a gas-release opening in the metal-transfer conduit enables pressure from the molten metal stream to assist in drawing gas into the molten metal stream. Such a structure and method is disclosed in U.S. application Ser. No. 10/773,101 entitled “System for Releasing Gas into Molten Metal”, invented by Paul V. Cooper, and filed on Feb. 4, 2004, the disclosure of which is incorporated herein by reference.
Molten metal transfer pumps have been used, among other things, to transfer molten aluminum from a well to a ladle or launder, wherein the launder normally directs the molten aluminum into a ladle or into molds where it is cast into solid, usable pieces, such as ingots.
Various filters have been utilized to remove impurities from the molten metal. These include single or double vertical gate filters, bonded particle filters, and cartridge filters. Vertical gate filters serve as walls between hearths and dip-out wells in melting or holding furnaces to remove inclusions. Bonded particle filtration can be used to remove oxides from molten aluminum and its alloys. Bonded particle filters may be in cylindrical, plate or custom shapes, and are available in various porosity and multiple grit levels.
First, known filters are often required to be semi-permanently cemented in place. This, results in increased maintenance time to remove and replace the filter. Also, relatively fragile bonded or sintered materials can break apart from the filter during use, which can result in inclusions that enter the molten metal being pumped and negatively affect the finished product or adversely affect pumping equipment.